The invention relates to a niobium-stabilized 14% chromium ferritic steel, and to use of same in the automobile sector.
The steels used for parts situated upstream from an exhaust system of a motor vehicle, the hot part of the system, must have both good resistance to oxidation and good creep resistance. Good formability is also necessary for manufacture of the manifold. The steels used for these hot parts are often either austenitic steels, which are relatively expensive and have poor oxidation resistance, although their formability is good, or bistabilized ferritic steels. The bistabilized ferritic steels have good oxidation resistance but are relatively difficult to form.
One goal of the invention is to provide an economical ferritic steel which exhibits very good resistance to both creep and oxidation at temperatures up to 1000xc2x0 C. as well as improved hardness for forming purposes.
The object of the invention is realized with a process for producing a sheet-metal strip of niobium-stabilized 14% chromium ferritic steel, wherein steel with the following composition by weight based on total weight:
carbonxe2x89xa60.02%,
0.002%xe2x89xa6nitrogenxe2x89xa60.02%,
0.05%xe2x89xa6siliconxe2x89xa61%,
0% less than manganesexe2x89xa61%,
0.2%xe2x89xa6niobiumxe2x89xa60.6%,
13.5%xe2x89xa6chromiumxe2x89xa616.5%,
0.02%xe2x89xa6molybdenumxe2x89xa61.5%,
0% less than copperxe2x89xa61.5%,
0% less than nickelxe2x89xa60.2%,
0% less than phosphorusxe2x89xa60.020%,
0% less than sulfurxe2x89xa60.003%,
0.005% less than tinxe2x89xa60.04%,
impurities inherent to smelting and iron
wherein the content of niobium, carbon and nitrogen satisfy the relationship:
9.5xe2x89xa6Nb/(C+N),
is subjected to:
reheating before hot rolling at a temperature of between 1150xc2x0 C. and 1250xc2x0 C. and preferably at about (xc2x115xc2x0 C.) 1175xc2x0 C.,
coiling at a temperature of between 600xc2x0 C. and 800xc2x0 C. and preferably of about (xc2x115xc2x0 C.) 600xc2x0 C.,
cold rolling of the coil with or without preliminary annealing,
final annealing of the sheet-metal strip at a temperature of between 800xc2x0 C. and 1100xc2x0 C. for a duration of between 1 minute and 5 minutes and preferably at a temperature of about (xc2x115xc2x0 C.) 1050xc2x0 C. for a time of about (xc2x115 sec) 2 minutes.
The other optional characteristics of the invention are:
after final annealing or before use, the sheet metal is subjected to heat treatment at a temperature of between 800xc2x0 C. and 1000xc2x0 C. for a time of between 1 minute and 100 hours and preferably at a temperature of about (xc2x115xc2x0 C.) 850xc2x0 C. for a time equal to or less than 30 minutes.
The invention also relates to a niobium-stabilized 14% chromium ferritic steel comprising, consisting of, and consisting essentially of, iron and the following by weight based on total weight:
carbonxe2x89xa60.02%,
0.002%xe2x89xa6nitrogenxe2x89xa60.02%,
0.05%xe2x89xa6siliconxe2x89xa61%,
0% less than manganesexe2x89xa61%,
0.2%xe2x89xa6niobiumxe2x89xa60.6%,
3.5%xe2x89xa6chromiumxe2x89xa616.5%,
0.02%xe2x89xa6molybdenumxe2x89xa61.5%, 0% less than copperxe2x89xa61.5%, 0% less than nickelxe2x89xa60.2%, 0% less than phosphorusxe2x89xa60.020%, 0% less than sulfurxe2x89xa60.003%, 0.005% less than tinxe2x89xa60.04%,
impurities inherent to smelting,
wherein the content of niobium, carbon and nitrogen satisfy the relationship:
9.5xe2x89xa6Nb/(C+N).
Other optional characteristics of the invention are:
the Nb content satisfies the relationship 0.1xe2x89xa6xcex94Nbxe2x89xa60.5, where xcex94Nb=Nbxe2x88x927(C+N) and preferably 0.2xe2x89xa6xcex94Nbxe2x89xa60.3,
the contents of niobium, silicon and molybdenum satisfy the relationship: xcex94Nb/(Si+Mo)xe2x89xa60.9 ,
the contents by weight of niobium and tin satisfy the relationship:
xcex94Nb/Snxe2x89xa650,
the contents of manganese and silicon satisfy the relationship:
Si/Mn greater than 1.
The contents of niobium, titanium, zirconium and aluminum satisfy the relationship:
Nb/(Ti+Zr+Al) greater than 0.16,
after heat treatment, the steel contains an intermetallic phase of Fe2Nb3 type with tetragonal structure at the grain boundaries.
The invention also relates to use of the ferritic steel sheet metal in the automobile sector, particularly for production of exhaust system manifolds.